The invention relates to UV- and heat-curable polyurethane dispersions which have UV-polymerizable Cxe2x95x90C double bonds, hydroxyl groups and blocked isocyanate groups in one molecule or the blocked isocyanate groups in admixed compounds, and to processes for their preparation and use.
Radiation-curable polyurethane dispersions are known, for example, from the Applicant""s DE-A 4434554 and are prepared from polyisocyanates, hydroxyl-containing polyesters, compounds containing an isocyanate-reactive group and an acid group, and compounds containing an isocyanate-reactive group and Cxe2x95x90C double bonds. In terms of their processing properties, however, the products leave a certain amount to be desired. U.S. Pat. No. 5,859,135 describes aqueous coating mixtures comprising a lipophilic polymer containing at least one hydroxyl group, having a molecular weight of up to 100,000, which is connected on one side to a crosslinkable functional group and on the other side, via a polyisocyanate, to a group containing carboxyl groups and to a hydrophilic polyalkylene oxide monoether radical.
A disadvantage is that coatings produced therewith have a relatively high intrinsic hydrophilicity, even after processing. U.S. Pat. No. 5,296,529 describes a self-crosslinking resin containing carboxyl, hydroxyl, and blocked isocyanate groups, which is prepared from a) a copolymer of a vinyl monomer containing free and blocked isocyanate groups with a styrene and/or (meth)acrylate comonomer, and b) a polyester resin containing hydroxyl and carboxyl groups, some hydroxyl groups of the polyester resin being reacted with some free isocyanate groups of the vinyl copolymer, and remaining isocyanate groups then being blocked. It can be seen that a highly reproducible production of the system counters the risk of a premature unwanted crosslinking of the two polymers. Also, the system described contains no UV-curable double bonds, nor is any radiation curing thereof described.
DE-A-198 60 041 describes reaction products of a) polyisocyanates and b) low molecular mass hydroxy compounds containing Cxe2x95x90C double bonds, such as hydroxyalkyl(meth)acrylates or hydroxyalkyl vinyl ethers, the majority of which constitute allophanates of the polyisocyanates with the unsaturated alcohols. The low molecular mass reaction products, which are of low viscosity, have a high polymerizable Cxe2x95x90C double bond content in the molecule and can be both polymerized with UV radiation and cured thermally, with the participation of the isocyanate groups, or by exposure to water vapor, ammonia, or amines. An application in the form of aqueous dispersions is not described.
It is an object of the present invention to prepare both UV- and heat-curable aqueous polyurethane dispersions which give rise to coatings having good chemical resistance and good mechanical properties, in particular a high scratch resistance, which dry physically after treatment even at unexposed areas, and are suitable for exterior applications such as for an automobile finish.
We have found that this object is achieved with polyurethane dispersions synthesized essentially from
a) aliphatic polyisocyanates having an NCO functionality of from 2 to 4.5,
b) compounds containing at least one isocyanate-reactive group and at least one UV-polymerizable Cxe2x95x90C double bond,
c) aliphatic and/or cycloaliphatic compounds containing at least two hydroxyl, mercapto and/or primary and/or secondary amino groups, having a molecular weight of less than 500 g/mol,
d) compounds containing at least one isocyanate-reactive group and also at least one carboxyl group or sulfonic acid group,
e) at least one basic compound for full or partial neutralization of the acid groups of the compounds d),
f) an isocyanate blocking agent (f1) which converts isocyanate groups into blocked isocyanate groups with an unblocking temperature in the range from 70 to 160xc2x0 C., or addition of a compound (f2) containing blocked isocyanate groups, and
g) if desired, a compound different than compounds b) to f) which contains only one isocyanate-reactive group,
the reaction products of a) with b) and, if appropriate, c), prepared with a stoichiometric excess of NCO groups in relation to the OH groups, having been reacted with compounds d) and e) in amounts sufficient for dispersibility in aqueous medium, isocyanate groups having been converted into blocked isocyanate groups by reaction with isocyanate blocking agents (f1), or compounds containing blocked isocyanate groups (f2) having been added.
The building blocks of the polyurethane dispersion:
Component a)
Aliphatic polyisocyanates a) having an NCO functionality of from 2 to 4.5 and preferably from 2.0 to 3.5 include especially, as aliphatic (including cycloaliphatic)diisocyanates, hexamethylene diisocyanate, 1,3- and 1,4-bis(isocyanatomethyl)cyclohexane, isophorone diisocyanate, and di(isocyanatocyclohexyl)methane. Preference is given to polyisocyanates having a functionality of at least 3, such as polyisocyanates containing isocyanurate groups, biuret groups, uretdione groups or urethane groups and/or allophanate groups. The polyisocyanates containing isocyanurate groups comprise, in particular, simple trisisocyanato isocyanurates, which constitute cyclic trimers of the diisocyanates, or mixtures thereof with their higher homologs containing more than one isocyanurate ring. The isocyanato isocyanurates generally have an NCO content of from 10 to 30, in particular from 15 to 25% by weight, and an average NCO functionality of 3 to 4.5. Polyisocyanates containing biuret groups are adducts of 3 molecules of diisocyanate with 1 molecule of water and have in particular an NCO content from 18 to 22% by weight and an average NCO functionality of from 3 to 4.5. Polyisocyanates containing urethane groups and/or allophanate groups may be obtained, for example, by reacting excess amounts of diisocyanate with simple alcohols such as trimethylolpropane, glycerol, 1,2-dihydroxypropane or mixtures thereof, for example, and generally have an NCO content of from 12 to 20% by weight and an average NCO functionality of from 2.5 to 3. Polyisocyanates having an NCO functionality of more than 2 that may be mentioned also include the adducts of 3 mol of diisocyanates such as isophorone diisocyanate with trihydric alcohols such as trimethylolpropane.
The component a) preferably comprises at least one compound Va) having two free isocyanate groups, at least one allophanate group, and at least one free-radically polymerizable Cxe2x95x90C double bond, a carbonyl group or an oxygen atom in ether function being attached directly to the double bond.
The compound Va) is preferably selected from compounds of the formula I
xe2x80x83OCNxe2x80x94R1"Parenopenst"R2xe2x80x94C(O)xe2x80x94R2xe2x80x94R1"Parenclosest"nNCOxe2x80x83xe2x80x83(I)
where
n is an integer from 1 to 10,
R1 is a divalent aliphatic or alicyclic C2 to C20 hydrocarbon unit or an aromatic C6 to C20 hydrocarbon unit,
R2 in each repeating unit is once xe2x80x94NHxe2x80x94 and once 
xe2x80x83R3 being a radical derived from an alcohol A by abstracting the hydrogen atom from the alcoholic hydroxyl group, said alcohol A further comprising at least one free-radically polymerizable Cxe2x95x90C double bond, and a carbonyl group or an oxygen atom in ether attachment being attached directly to the double bond.
The radicals R1 preferably comprise those derived by abstracting the isocyanate group from customary aliphatic or aromatic polyisocyanates. The diisocyanates are preferably aliphatic isocyanates having 4 to 20 carbon atoms. Examples of customary diisocyanates are aliphatic diisocyanates such as tetramethylene diisocyanate, hexylmethylene diisocyanate, octamethylene diisocyanate, decamethylene diisocyanate, dodecamethylene diisocyanate, tetradecamethylene diisocyanate, derivatives of lysine diisocyanate, tetramethylxylylene diisocyanate, cycloaliphatic diisocyanates such as 1,4-, 1,3- or 1,2-diisocyanatocyclohexane, 4,4xe2x80x2- or 2,4xe2x80x2-di(isocyanatocyclohexyl)methane, isophorone diisocyanate, 1,3- or 1,4-bis(isocyanatomethyl)cyclohexane, 2,4- and 2,6-diisocyanato-1-methylcyclohexane, and also aromatic diisocyanates such as 2,4- or 2,6-tolylene diisocyanate, m- or p-xylylene diisocyanate, 2,4xe2x80x2- or 4,4xe2x80x2-diisocyanatodiphenylmethane, 1,3- or 1,4-phenylene diisocyanate, 1-chloro-2,4-phenylene diisocyanate, 1,5-naphthylene diisocyanate, diphenylene 4,4xe2x80x2-diisocyanate, 4,4xe2x80x2-diisocyanato-3,3xe2x80x2-dimethyldiphenyl diisocyanate, 3-methyldiphenylmethane 4,4xe2x80x2-diisocyanate, and diphenyl ether 4,4xe2x80x2-diisocyanate. Mixtures of the abovementioned diisocyanates may be present. Preference is given to hexamethylene diisocyanate, 1,3-bis(isocyanatomethyl)-cyclohexane, isophorone diisocyanate, tetram thylxylylene diisocyanate, and di(isocyanatocyclohexyl)methane.
The alcohols A from which the radical R3 is derived comprise, for example, esters of xcex1,xcex2-unsaturated carboxylic acids, such as acrylic acid, methacrylic acid (below for short xe2x80x9c(meth)acrylic acidxe2x80x9d), crotonic acid, acrylamidoglycolic acid, methacrylamidoglycolic acid or vinylacetic acid and polyols having preferably 2 to 20 carbon atoms and at least 2 hydroxyl groups, such as ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, dipropylene glycol, tripropylene glycol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 2-methyl-1,5-pentanediol, 2-ethyl-1,4-butanediol, 1,4-dimethylolcyclohexane, glycerol, trimethylolethane, trimethylolpropane, trimethylolbutane, penta-erythritol, ditrimethylolpropane, erythritol and sorbitol, provided the ester has at least one isocyanate-reactive OH group. The radicals R3 may also be derived from the amides of (meth)-acrylic acid with amino alcohols, e.g. 2-aminoethanol, 3-amino-1-propanol, 1-amino-2-propanol or 2-(2-aminoethoxy)-ethanol, and the vinyl ethers of the abovementioned polyols, provided they still have one free OH group.
Also suitable as reactive components, furthermore, are unsaturated polyetherols or polyesterols or polyacrylate polyols having an average OH functionality of from 2 to 10.
The radicals R3 are preferably derived from alcohols such as 2-hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate, 1,4-butanediol mono(meth)acrylate, neopentyl glycol mono(meth)acrylate, glycerol mono- and di(meth)acrylate, trimethylolpropane mono- and di(meth)acrylate, and penta-erythritol di- and tri(meth)acrylate. With particular preference, the alcohol A is selected from 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate and hydroxypropyl(meth)acrylate. Examples of amides of ethylenically unsaturated carboxylic acids with amino alcohols are hydroxyalkyl(meth)acrylamides such as N-hydroxymethyl(meth)acrylamide, N-hydroxyethyl(meth)acrylamide, 5-hydroxy-3-oxopentyl(meth)acrylamide, N-hydroxyalkylcrotonamides such as N-hydroxymethylcrotonamide or N-hydroxyalkylmaleimides such as N-hydroxyethylmaleimide.
In particular the component a) comprises at least one compound Va) and at least one further, different aliphatic or araliphatic polyisocyanate. Preferred further polyisocyanates are polyisocyanates having an NCO functionality of from 2 to 4.5, with particular preference from 2 to 3.5. It is preferred to use aliphatic, cycloaliphatic, and araliphatic diisocyanates. These include, for example, tetramethylene diisocyanate, hexamethylene diisocyanate, 2,3,3-trimethylhexamethylene diisocyanate, 1,4-cyclohexylene diisocyanate, isophorone diisocyanate, 1,3- and 1,4-bis(isocyanatomethyl)cyclohexane, di(isocyanatocyclohexyl)methane, tetramethylxylylene diisocyanate, and mixtures thereof. Preference is given to polyisocyanates containing not only two or more isocyanate groups but also a group selected from the group of the urethane, urea, biuret, allophanate, carbodiimide, uretonimine, uretdione, and isocyanurate groups. Preferred additional polyisocyanates are isophorone diisocyanate, 1,3- and 1,4-bis(isocyanatomethyl)cyclohexane, their isocyanurates, biurets and allophanates, and mixtures thereof.
Particularly preferred polyisocyanate starting materials for preparing the curable polyurethane dispersions of the invention are allophanate-group-containing polyisocyanates of hexamethylene diisocyanate or of isophorone diisocyanate with the hydroxy compounds b) containing Cxe2x95x90C groups, as described in DE-A-198 60 041, and especially the corresponding reaction products of the polyisocyanates with the hydroxyalkyl (meth)acrylates.
Component b)
The compounds of component b) generally contain a free-radically polymerizable Cxe2x95x90C double bond and also at least one other isocyanate-reactive group. Examples of preferred compounds of components b) are the monoesters of dihydric or polyhydric alcohols with xcex1,xcex2-ethylenically unsaturated mono- and/or dicarboxylic acids and their anhydrides. xcex1,xcex2-Ethylenically unsaturated mono- and/or dicarboxylic acids and their anhydrides which may be used include, for example, acrylic acid, methacrylic acid, fumaric acid, maleic acid, maleic anhydride, crotonic acid, itaconic acid, etc. It is preferred to use acrylic acid and methacrylic acid. Examples of suitable alcohols are diols such as glycols, preferably glycols of 2 up to 25 carbon atoms, such as 1,2-ethanediol, 1,3-propanediol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,4-pentanediol, 1,6-hexanediol, 1,10-decanediol, diethylene glycol, etc. Suitable triols and polyols have, for example, 3 to 25, preferably 3 to 18, carbon atoms. Examples include glycerol, trimethylolpropane, erythritol, pentaerythritol, sorbitol, etc. The compounds of component b) are selected preferably from 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, 3-hydroxybutyl acrylate, 3-hydroxybutyl methacrylate, 4-hydroxybutyl acrylate, 4-hydroxybutyl methacrylate, 6-hydroxyhexyl acrylate, 6-hydroxyhexyl methacrylate, 3-hydroxy-2-ethylhexyl acrylate, 3-hydroxy-2-ethylhexyl methacrylate, trimethylolpropane monoacrylate and mixtures thereof. If desired, these compounds may also have been chain-extended by reaction with an appropriate chain extender, such as a polyfunctional isocyanate or a polyfunctional carboxylic acid, for example.
Further suitable compounds b) are the esters and amides of amino alcohols with the abovementioned xcex1,xcex2-ethylenically unsaturated mono- and/or dicarboxylic acids, hydroxyalkyl vinyl ethers such as hydroxybutyl vinyl ether, etc.
Particularly suitable compounds b) containing at least one isocyanate-reactive group and also at least one Cxe2x95x90C double bond polymerizable with UV radiation in the presence of a photoinitiator are methacrylic monoesters and acrylic monoesters of aliphatic diols and also methacrylamides and acrylamides of amino alcohols, and, furthermore, hydroxyalkyl vinyl ethers such as hydroxybutyl vinyl ether, of which preference is given to hydroxyalkyl acrylates having 2 to 4 carbon atoms in the alkyl radical, such as 2-hydroxyethyl acrylate, where the adjacent carbonyl group or ether group contributes to activating the Cxe2x95x90C double bond.
Component c)
For the preparation of the polyurethane it is advantageous to use further aliphatic compounds c) containing at least two isocyanate-reactive hydroxyl, mercapto and/or amino groups and having a molecular weight of less than 500 g/mol. Particularly suitable such compounds are hydrolytically stable short-chain diols such as dihydroxymethylcyclohexane, bis(hydroxycyclohexyl)-propane, tetramethylcyclobutanediol, cyclooctanediol, or norbornanediol. Preference is given to the use of hydrocarbon diols having a C number of from 6 to 20, such as hexanediol, octanediol, decanediol, or dodecanediol.
The additional use of polyfunctional alcohols, amino alcohols or thio alcohols may also serve, however, to introduce isocyanate-reactive functional groups prior to final curing, if the stoichiometry is chosen so that statistically only a few of the isocyanate-reactive groups react in reactions prior to final curing. To accelerate the reaction of the polyisocyanates it is possible to use the customary catalysts such as dibutyltin dilaurate, tin(II) octoate, or diazabicyclo[2.2.2]octane. In the preparation of allophanates of the polyisocyanates with the unsaturated alcohols at temperatures of from 20 to 280xc2x0 C., the presence of catalysts which promote allophanate formation is advantageous, such as organozinc compounds or tetraalkylammonium compounds. Regarding the preparation of the allophanates, reference may be made again to the remarks in DE-A-198 60 041.
Component d)
Particularly suitable compounds d) containing at least one isocyanate-reactive group and also at least one carboxylic acid or sulfonic acid group are aliphatic monomercapto-, monohydroxy- and monoamino- and iminocarboxylic acids and corresponding sulfonic acids, such as mercaptoacetic acid (thioglycolic acid), mercaptopropionic acid, mercaptosuccinic acid, hydroxyacetic acid, hydroxypivalic acid, dimethylolpropionic acid, hydroxydecanoic acid, hydroxydodecanoic acid, 12-hydroxystearic acid, hydroxyethanesulfonic acid, hydroxypropanesulfonic acid, mercaptoethanesulfonic acid, mercaptopropanesulfonic acid, aminopropanesulfonic acid, glycine (aminoacetic acid), or iminodiacetic acid.
Component e)
Suitable basic compounds e) for full or partial neutralization of the acid groups of the compounds include organic and inorganic bases such as alkali metal and alkaline earth metal hydroxides, oxides, carbonates, hydrogen carbonates, and also ammonia or primary, secondary, or tertiary amines. Preference is given to full or partial neutralization with amines such as with ethanolamine or diethanolamine and in particular with tertiary amines, such as triethylamine, triethanolamine, dimethylethanolamine, or diethylethanolamine. The amounts of chemically bonded acid groups introduced and the extent of the neutralization of the acid groups (which is usually from 40 to 100% of the equivalence basis) should be sufficient to ensure dispersion of the polyurethanes in an aqueous medium, as is familiar to the skilled worker.
Component f)
Suitable blocking agents for isocyanate groups (f1) are compounds which convert the isocyanate groups into blocked isocyanate groups which subsequently below their unblocking temperature do not exhibit the customary reactions of a free isocyanate group. When the blocked isocyanate groups are heated to a temperature which corresponds at least to the unblocking temperature, which for the purposes of this invention is to be situated within the range from 70 to 160xc2x0 C., the isocyanate groups are exposed again and are available for customary isocyanate reactions, for example, for reactions with functional groups such as hydroxyl, mercapto, or amino groups, for example. Compounds which block (cap, mask or protect) the isocyanate groups have been widely described in the literature (cf., e.g., Z. W. Wicks, Prog. Org. Coat. 3(1975) 73-99 and also 9(1981) 3-28 or Houben-Weyl, Methoden der Organischen Chemie Vol. XIV/2, p. 61 ff., Georg Thieme Verlag, Stuttgart 1963). Typical blocking agents of isocyanate groups (f1) are phenols, caprolactam, imidazoles, pyrazoles, pyrazolinones, 1,2,4-triazoles, diketopiperazines, malonates, and oximes. Preference is given to oximes such as 2-butanone oxime, 3,5-dimethylpyrazole, and 1,2,4-triazoles. Instead of blocking isocyanate groups in the same molecule with the blocking agent, which is preferred, an alternative is to admix to the polyurethane a low molecular mass compound containing blocked isocyanate groups (f2), which subsequently, in the course of curing, with heating to at least the unblocking temperature, exposes reactive isocyanate groups and is available for the curing or crosslinking reaction. It is also possible to use mixtures of isocyanate blocking agents having different unblocking temperatures within the aforementioned range.
Component g)
Finally, it is possible to use compounds g) which are different than the compounds b) to f) and have only one isocyanate-reactive group, in order, for example, to modify the properties of the polyurethane and/or to reduce the number of reactive free isocyanate groups.
In order to modify the polyurethane dispersions of the invention they may be admixed conventionally with reactive diluents, as described in P.K.T. Oldring (Editor), Chemistry and Technology of UV and EB Formulations for Coatings, Inks and Paints, Vol. II, Chapter III: Reactive Diluents for UV and EB Curable Formulations, Wiley and SITA Technology, London 1997. In accordance with a first embodiment, the reactive diluents are to contain no hydroxyl groups. Preferred reactive diluents are esters of acrylic acid which are derived from diols or polyols, preferably aliphatic polyhydric polyalcohols and alkoxylation products thereof. Examples of very suitable reactive diluents which may also be used for a further increase in hardness of the coatings produced with the polyurethane dispersions are, for example, trimethylolethane tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, or pentaerythritol tetra(meth)acrylate. Preference is also given to hexanediol diacrylate, hexanediol dimethacrylate, octanediol diacrylate, octanediol dimethacrylate, nonanediol diacrylate, nonanediol dimethacrylate, decanediol diacrylate, and decanediol dimethacrylate. Further suitable compounds are the esters of alicyclic diols, such as cyclohexanediol di(meth)acrylate and bis(hydroxymethylethyl)cyclohexane di(meth)acrylate. Other suitable reactive diluents include trimethylolpropane monoformal acrylate, glycerol formal acrylate, 4-tetrahydropyranyl acrylate, 2-tetrahydropyranyl methacrylate and tetrahydrofurfuryl acrylate.
In accordance with another embodiment, the reactive diluents have at least two functional groups, selected from free-radically polymerizable double bonds and isocyanate-reactive groups. They include, on the one hand, polymeric polyols other than component c). The number-average molecular weight Mn of these polymers is preferably situated within a range from about 1000 to 100,000, with particular preference from 2000 to 10,000. The OH numbers are preferably within a range from about 40 to 200 mg KOH/g polymer. Preferred polymers are, for example, copolymers containing in copolymerized form at least one of the abovementioned monoesters of dihydric or polyhydric alcohols with at least one xcex1,xcex2-ethylenically unsaturated mono- and/or dicarboxylic acid and at least one further comonomer selected preferably from vinylaromatic compounds, such as styrene, for example, esters of the abovementioned xcex1,xcex2-unsaturated mono- and/or dicarboxylic acids with monoalcohols, vinyl esters of carboxylic acids containing up to 20 carbon atoms, vinyl halides, nonaromatic hydrocarbons having 4 to 8 carbon atoms and 1 or 2 double bonds, unsaturated nitrites, etc., and mixtures thereof. They further include (partially) hydrolyzed vinyl ester polymers, preferably polyvinyl acetates. They additionally include polyesterols based on aliphatic, cycloaliphatic and/or aromatic di-, tri- and/or polycarboxylic acids with di-, tri- and/or polyols and also lactone-based polyesterols. They additionally include polyetherols obtainable by polymerizing cyclic ethers or by reacting alkylene oxides with a starter molecule, and also xcex1,xcfx89-diamino polyethers obtainable by reacting polyetherols with ammonia. Moreover, they also include customary polycarbonates known to the skilled worker and containing terminal hydroxyl groups, which are obtainable, for example, by reacting the aforementioned diols with phosgene or carbonic diesters.
Suitable reactive diluents containing at least one free-radically polymerizable Cxe2x95x90C double bond and at least one isocyanate-reactive group are the esters and polyesters of the aforementioned xcex1,xcex2-ethylenically unsaturated mono- and/or dicarboxylic acids with diols or polyols that still contain free hydroxyl groups. They include, for example, pentaerythritol diacrylate, dipentaerythritol tetraacrylate, dipentaerythritol triacrylate, etc. Also suitable are the esters, again still containing free hydroxyl groups, of alkoxylated polyols with xcex1,xcex2-ethylenically unsaturated mono- and/or dicarboxylic acids, such as the acrylates or methacrylates of alkoxylated trimethylolpropane, glycerol, or pentaerythritol, for example.
The coating compositions of the invention may be prepared from the individual components described and by the above instructions in accordance with techniques customary to the skilled worker, possibly using known coating additives, such as leveling agents, defoamers, UV absorbers, dyes, pigments and/or fillers.
The polyurethane content of the aqueous dispersions may be in particular between 5 and 70 and in particular between 20 and 50% by weight, the solids content being determined gravimetrically.
Prior to the curing of the polyurethane dispersions with UV radiation, it is advantageous to add to them photoinitiators in an amount of from 0.01 up to 10 and preferably from 1 to 5% by weight, based on the solids content of the dispersion, these photoinitiators being as specified in Patent Application DE-A-198 60 041.
Examples hereof include benzophenone and benzophenone derivatives, such as 4-phenylbenzophenone and 4-chlorobenzophenone, Michler""s ketone, acetophenone derivatives, such as 1-benzoylcyclohexan-1-ol, 2-hydroxy-2,2-dimethylacetophenone, and 2,2-dimethoxy-2-phenylacetophenone, benzoin and benzoin ethers, such as methyl, ethyl and butylbenzoin ethers, benzil ketals, such as benzil dimethyl ketal, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one, anthraquinone and its derivatives, such as methylanthraquinone and tert-butylanthraquinone, acylphosphine oxides, such as 2,4,6-trimethylbenzoyldiphenylphosphine oxide, ethyl 2,4,6-trimethylbenzoylphenylphosphinate, methyl 2,4,6-trimethylbenzoylphenylphosphinate and bisacylphosphine oxides.
For particular preference the dispersions of the invention include at least one photoinitiator selected from phenylglyoxalic acid and the esters and salts thereof. Particular preference is given to compounds of the formula I 
R1 is a hydrogen atom or a C1-C18 alkyl group. Preferably, R1 is a C1-C8 alkyl group, especially methyl, ethyl, propyl, butyl, or hexyl.
R2 and R3 independently of one another are a hydrogen atom or a C1-C18 alkyl or C1-C18 alkoxy group.
Preferably R2 and R3 independently of one another are a hydrogen atom.
Where at least one of the two radicals R2 and R3 is other than a hydrogen atom, the phenyl ring is substituted preferably para (in position 4) to the carbonyl group.
Particular preference is also given to phenylglyoxalic esters of the formula II 
where the two radicals R4 independently of one another are a radical of the formula 
R5, RS and R7 independently of one another are H, C1-C6 alkyl unsubstituted or substituted by OH, OC1-C6 alkyl or OCOC1-C6 alkyl, or are OH or OC1-C6 alkyl;
A is C2-C6 alkylene or a radical of the formulae 
the radicals R8 independently of one another are H or COCOR4, and A1 is C2-C6 alkylene or 
Compounds of this kind are described in DE-A-198 26 712 and German Patent Application P-199 13 353.0, the entirety of which is incorporated here by reference. Preferably, the above-described photoinitiators based on phenylglyoxalic acid derivatives are suitable for exterior applications, since they show little or no yellowing.
In accordance with one suitable embodiment, the dispersions of the invention further comprise at least one thermal initiator. Preferred thermal initiators are those having a half-life at 60xc2x0 C. of at least one hour, preferably at least two hours. The half-life of a thermal initiator is the time at which half of the initial amount of the initiator has undergone decomposition into free radicals. On a substrate coated with a dispersion of the invention, these initiators generally permit the formation of a film by customary methods, such as evaporation with heating, for example, at which point essentially no thermal initiation and curing yet takes place.
The thermal initiator component is used preferably in an amount of from 0.1 to 10% by weight, more preferably from 0.5 to 5% by weight, based on the total amount of components a) to g).
Suitable thermal initiators are, generally, all compounds which undergo decomposition into free radicals under the curing conditions, such as, for example, peroxides, hydroperoxides, hydrogen peroxides, persulfates, azo compounds, highly substitutedxe2x80x94e.g., hexasubstitutedxe2x80x94ethanes, amine N-oxides, redox catalysts, etc. Preference is given to the use of water-soluble initiators. Examples of suitable thermal initiators are triphenylmethylazobenzene, benzoyl peroxide, di-tert-butyl peroxide, cumene hydroperoxide, dicumyl peroxide, tert-butyl perbenzoate, 2,2,6,6-tetramethylpiperidin-1-yloxy, benzpinacol and derivatives thereof.
The thermal initiator component further comprises, preferably, at least one thermal initiator containing at least one isocyanate-reactive group, the initiator also being capable, after reaction with a compound containing isocyanate groups, of releasing free radicals under heat. These include, for example, initiators containing at least one bydroxyl group by means of which they may be incorporated into the polymer.
Preference is given to hexasubstituted ethanes, especially benzpinacol and the derivatives thereof, silylated pinacols, available commercially, for example, under the trade name ADDID 600 from Wacker, or hydroxyl-containing amine N-oxides, such as 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-yloxy (TEMPOL).
Preference is given to dispersions in which, of the isocyanate groups of the compounds of component a),
from 0 to 70 mol %, preferably from 20 to 60 mol %, have been reacted with isocyanate-reactive groups of at least one compound of component c), and
from 2 to 10 mol %, preferably from 4 to 8 mol %, have been reacted with isocyanate-reactive groups of at least one compound of component d).
The remainder of the isocyanate groups, up to 100 mol %, essentially comprise blocked isocyanate groups. The ratio of blocked NCO equivalents to free, NCO-reactive groups is preferably from about 2:1 to 1:2, in particular about 1:1.
DE-A-198 60 041 also describes the implementation of the coating of the substrates, the implementation of the UV curing, which is advantageously effected under an inert gas atmosphere, and the implementation of the thermal (heat) curing, where the addition of peroxide has been found advantageous. The coating techniques specified in German Patent Application DE-A-198 60 041 also apply, mutatis mutandis, to coatings with the polyurethane dispersions of the invention.
The substrates are generally coated by conventional techniques, known to the skilled worker, in which at least one dispersion of the invention is applied in the desired thickness to the target substrate and the volatile constituents of the dispersions are removed. If desired, this procedure may be repeated one or more times. Application to the substrate can be made in a known manner, for example, by spraying, troweling, knifecoating, brushing, rolling, rollercoating or flowcoating. The coating thickness is generally in a range from about 3 to 1000 g/m2 and, preferably, from 10 to 200 g/m2.
In general, the films formed on the substrate are cured by exposure to high-energy radiation, and thermally. The sequence of the curing steps is arbitrary.
If desired, if two or more coats of the coating composition are applied over one another, radiation curing may take place after each coating operation.
Radiation curing takes place preferably by exposure to high-energy radiation, i.e., UV radiation, or daylight, preferably light with a wavelength from 250 to 600 nm, or by bombardment with high-energy electrons (electron beams; from 150 to 300 keV). Examples of the radiation sources used are high-pressure mercury vapor lamps, lasers, pulsed lamps (flashlights), halogen lamps, or excimer emitters. In the case of UV curing, the radiation dose which is usually sufficient for crosslinking is in the range from 80 to 3000 mJ/cm2.
If desired, exposure may also take place in the absence of oxygen, e.g., under an inert gas atmosphere. Suitable inert gases are preferably nitrogen, noble gases, carbon dioxide, or combustion gases. Exposure may also take place with the coating composition covered with transparent media. Examples of transparent media are polymer films, glass, or liquids, e.g., water.
In one preferred technique curing is carried out continuously by passing the substrate, treated with the formulation of the invention, at constant speed past a radiation source. For this technique, the curing rate of the formulation of the invention needs to be sufficiently high.
This difference in the progress of curing over time may be exploited in particular when the coating of the article is followed by another processing step in which the film surface enters into direct contact with another article or is worked mechanically.
The advantage of the dispersions of the invention is that the coated articles may be processed further directly following radiation curing, since the surface no longer sticks. On the other hand, the precured film is still sufficiently flexible and extensible that the article can be deformed without the film flaking or tearing.
Even if deformation of the article is not intended, the technique, known as dual cure, may prove advantageous, because the articles provided with the precured film are particularly easy to transport and store, in stacks, for example. Moreover, the dual cure technique offers the advantage that the coating compositions are able to undergo chemical aftercuring in dark regions (regions unaccessible to radiation) and, consequently, adequate material properties may still be achieved independently of irradiation. Furthermore, spray mist deposits undergo tack-free and emissions-free curing.
The invention additionally provides for the use of a dispersion, as described above, to coat substrates of metal, wood, paper, ceramic, glass, plastic, textile, leather, nonwoven, or mineral building materials.
With particular preference, the dispersions of the invention are suitable as or in exterior coatings, preferably of buildings or parts of buildings, road markings, and coatings on motor vehicles and aircraft.
The polyurethane dispersions of the invention are suitable with particular advantage for coating substrates which can be coated at temperatures up to 160xc2x0 C., especially metallic substrates such as iron or aluminum. The coating compositions of the invention exhibit particular advantages in connection with their use as automotive clearcoats.
Relative to similar known products, the aqueous polyurethane dispersions prepared in accordance with the invention exhibit high scratch resistance in combination with good chemical resistance, good weathering stability and good mechanical properties, and also good coatings properties in the unexposed regions.
The purpose of the examples which follow is to illustrate the invention, but not restrict it.